As designers turn their attention to mobile WiMAX devices, they are quickly learning that there are some specific design challenges regarding power amplifiers. For Wave 2 mobile WiMAX products, the mobile device needs to efficiently deliver +23 dBm output power with high linearity from a 3.3 VDC supply.
Managing power in mobile WiMAX is quickly shaping up to be vitally important as first-generation designs are tested and deployed. One of the challenges of designing for mobile WiMAX is its long range, since WiMAX networks typically achieve coverage of about 1 km per cell.
To achieve these ranges, WiMAX must have an optimized power profile—from the base station right down to the components in the mobile device. High transmit power, then, is important. But how high can WiMAX go and what are the limitations imposed by regulatory bodies, technological limits, and usage models?
Designers of the power amplifier (PA) and those selecting PAs need to find the optimal balance between high power and high efficiency in order to ensure robust links, high data rates, and good range for their WiMAX services.
The nature of WiMAX
What makes WiMAX challenging for designers is that it is an access technology with a unique set of constraints. As a result, power amplification circuits that were used for cellular or Wi-Fi applications cannot simply be dropped into WiMAX designs and tweaked to perform adequately.
In many ways, WiMAX can be considered a hybrid technology because it shares aspects of both cellular and Wi-Fi networks. Mobile WiMAX is very similar to cellular; it is meant to be used in highly mobile devices and it uses licensed frequency bands (so users expect high reliability). It also employs transmit power control techniques, much like CDMA cellular does.
However, it differs from cellular because it operates at much higher data rates (resulting in more stringent linearity requirements) and must simultaneously handle voice over Internet Protocol (VoIP), data, and video transmissions. Managing the bandwidth and priority of transmission for these types of services requires a quality of service (QoS) component that is not required for mobile voice alone.
On the other hand, WiMAX is also similar to Wi-Fi. For instance, it offers high data rates, uses orthogonal frequency division multiplexing (OFDM) with modulations from BPSK to 64-QAM, and is an all-IP-based network.
However, it differs from Wi-Fi because it uses a fully-scheduled service, unlike the collision-based carrier sense multiple access (CSMA) technique used by Wi-Fi. This gives WiMAX a significant advantage over Wi-Fi.
As the number of users increases in a CSMA network, overall capacity drops dramatically since each collision requires a subsequent retransmission. With a scheduled service, overall network capacity is unaffected as the number of users increases, since the basestation manages each user's access to the network efficiently.
